Understanding U-Values: What’s Changed in Part L 2023?

If you're planning a new home or extension, you'll need to meet updated Building Regulations under Part L. These changes affect how well your walls, floors, roofs, and windows keep the heat in—and the key number to watch is the U-value.

What is a U-Value?

A U-value tells you how much heat passes through a building material. The lower the number, the better it keeps your home warm. Part L sets maximum U-values for different parts of a building.

U-Value Comparison Table

Here’s how the new rules compare with the old ones:

Element	Pre-2023 (Old Rules)	New Builds (2023+)	Extensions (2023+)
Walls	≤ 0.18 W/m²K	≤ 0.16 W/m²K	≤ 0.18 W/m²K
Roofs	≤ 0.13 W/m²K	≤ 0.11 W/m²K	≤ 0.15 W/m²K
Floors	≤ 0.13 W/m²K	≤ 0.11 W/m²K	≤ 0.18 W/m²K
Windows & Doors	≤ 1.4 W/m²K	≤ 1.2 W/m²K	≤ 1.4 W/m²K

So if you're building new, the bar is higher—especially for glazing.

What Does This Mean for Your Project?

• For new homes, insulation and glazing must now perform to higher standards to meet tougher energy rules. In simple terms, that means there's less flexibility to include large areas of glass.

• For extensions, any new walls, floors, and roofs must meet the "extension" U-values above.

• Replacing windows or adding new ones? They need to be 1.4 W/m²K or better—triple glazing is often the easiest fix.

How BetterLivingSpace Ltd Can Help

We carry out accurate U-value assessments and advise you on how to meet Part L—whether it’s a new build or extension. We can recommend the right materials and insulation levels so you meet the latest energy rules without overdesigning.

Have a project coming up? Let’s chat about how to make it compliant—and comfortable.

Visit www.betterlivinspace.co.uk or call 03301 757 601.